A variety of processes are used to recover viscous hydrocarbons, such as heavy oils and bitumen, from underground deposits. There are extensive deposits of viscous hydrocarbons around the world, including large deposits in the Northern Alberta tar sands, that are not amenable to standard oil well production technologies. The primary problem associated with producing hydrocarbons from such deposits is that the hydrocarbons are too viscous to flow at commercially relevant rates at the temperatures and pressures present in the reservoir. In some cases, such deposits are mined using open-pit mining techniques to extract the hydrocarbon-bearing material for later processing to extract the hydrocarbons. Alternatively, thermal techniques may be used to heat the reservoir to produce the heated, mobilized hydrocarbons from wells. One such technique for utilizing a single horizontal well for injecting heated fluids and producing hydrocarbons is described in U.S. Pat. No. 4,116,275, which also describes some of the problems associated with the production of mobilized viscous hydrocarbons from horizontal wells.
One thermal method of recovering viscous hydrocarbons using two vertically spaced horizontal wells is known as steam-assisted gravity drainage (SAGD). Various embodiments of the SAGD process are described in Canadian Patent No. 1,304,287 and corresponding U.S. Pat. No. 4,344,485. In the SAGD process, steam is pumped through an upper, horizontal, injection well into a viscous hydrocarbon reservoir while hydrocarbons are produced from a lower, parallel, horizontal, production well vertically spaced proximate to the injection well. The injection and production wells are typically located close to the bottom of the hydrocarbon deposit.
It is believed that the SAGD process works as follows. The injected steam initially mobilises the in-place hydrocarbon to create a “steam chamber” in the reservoir around and above the horizontal injection well. The term “steam chamber” means the volume of the reservoir which is saturated with injected steam and from which mobilised oil has at least partially drained. As the steam chamber expands upwardly and laterally from the injection well, viscous hydrocarbons in the reservoir are heated and mobilized, especially at the margins of the steam chamber where the steam condenses and heats a layer of viscous hydrocarbons by thermal conduction. The mobilized hydrocarbons (and aqueous condensate) drain under the effects of gravity towards the bottom of the steam chamber, where the production well is located. The mobilized hydrocarbons are collected and produced from the production well. The rate of steam injection and the rate of hydrocarbon production may be modulated to control the growth of the steam chamber to ensure that the production well remains located at the bottom of the steam chamber in an appropriate position to collect mobilized hydrocarbons.
Alternative primary recovery processes may be used that employ thermal and non-thermal components to mobilize oil. For example, light hydrocarbons may be used to mobilize heavy oil. U.S. Pat. No. 5,407,009 teaches an exemplary technique of injecting a hydrocarbon solvent vapour, such as ethane, propane or butane, to mobilize hydrocarbons in the reservoir.
Heavy oil recovery techniques such as SAGD create mobile zone chambers in a reservoir, from which at least some of the original oil-in-place has been recovered. However, reservoirs depleted by such processes typically contain a significant volume of residual hydrocarbons. There remains a need for methods that may be used to recover these residual hydrocarbons.
In the context of the present application, various terms are used in accordance with what is understood to be the ordinary meaning of those terms. For example, “petroleum” is a naturally occurring mixture consisting predominantly of hydrocarbons in the gaseous, liquid or solid phase. In the context of the present application, the words “petroleum” and “hydrocarbon” are used to refer to mixtures of widely varying composition. The production of petroleum from a reservoir necessarily involves the production of hydrocarbons, but is not limited to hydrocarbon production. Similarly, processes that produce hydrocarbons from a well will generally also produce petroleum fluids that are not hydrocarbons. In accordance with this usage, a process for producing petroleum or hydrocarbons is not necessarily a process that produces exclusively petroleum or hydrocarbons, respectively. “Fluids”, such as petroleum fluids, include both liquids and gases.
It is common practice to segregate petroleum substances of high viscosity and density into two categories, “heavy oil” and “bitumen”. For example, some sources define “heavy oil” as a petroleum that has a mass density of greater than about 900 kg/m3. Bitumen is sometimes described as that portion of petroleum that exists in the semi-solid or solid phase in natural deposits, with a mass density greater than about 1000 kg/m3 and a viscosity greater than 10,000 centipoise (cP; or 10 Pa.s) measured at original temperature in the deposit and atmospheric pressure, on a gas-free basis. Although these terms are in common use, references to heavy oil and bitumen represent categories of convenience, and there is a continuum of properties between heavy oil and bitumen. Accordingly, references to heavy oil and/or bitumen herein include the continuum of such substances, and do not imply the existence of some fixed and universally recognized boundary between the two substances. In particular, the term “heavy oil” includes within its scope all “bitumen” including hydrocarbons that are present in semi-solid or solid form.
A reservoir is a subsurface formation containing one or more natural accumulations of moveable petroleum, which are generally confined by relatively impermeable rock. In a reservoir, the mobility of entrained fluids, such as petroleum, may be defined as the ratio of permeability to viscosity. The higher the permeability, all other things being equal, the higher the mobility. Correspondingly, the higher the viscosity, the lower the mobility. A “mobile zone” within a reservoir is a contiguous region characterised as having greater mobility than adjoining regions. The mobility of fluids within the mobile zone may vary, and some regions within a mobile zone may in fact exhibit less mobility than adjoining regions, while the mobile zone as a whole is nevertheless characterised as a region of relatively high mobility. Accordingly, the term “mobile zone” as used herein is a relative term, meaning that the zone referred to contains a fluid that is more mobile than fluids in adjoining zones.
An “oil sand” or “tar sand” reservoir is generally comprised of strata of sand or sandstone containing petroleum. Mobile zones may exist in oil sand or tar sand reservoirs, within or across strata, and may extend into adjoining strata.
A “chamber” within a reservoir or formation is a region that is in fluid communication with a particular well or wells, such as an injection or production well. For example, in a SAGD process, a steam chamber is the region of the reservoir in fluid communication with a steam injection well, which is also the region that is subject to depletion, primarily by gravity drainage, into a production well. In some processes, chambers that are in fluid communication with a single well may be enlarged so that they then communicate with additional wells or chambers, when this occurs it may be characterised as a “breakthrough” of fluid communication from one well to another, or a coalescence of steam chambers. In accordance with the foregoing, in the context of the present invention, a “mobile zone chamber” is a chamber that encompasses a mobile zone.
A wide variety of horizontal well drilling techniques are known. In typical horizontal wells, a single well will generally have segments that are primarily horizontal, as well as segments that are primarily vertical. In the context of the present invention, a generally horizontal well segment in a reservoir is a segment of a well that has a horizontal distance that is at least as great as its vertical distance within the hydrocarbon reservoir.